The service life of polyamides is dependent on (1) the raw materials used in the manufacturing process, (2) the additives used in stabilizing the polymer against damage brought about by melt-processing and (3) environmental weathering factors. A serious drawback to providing stabilization systems for polyamides lies in the fact that the maximum concentration of many additives is limited. Polyamides are heretofore regarded as poor in solvating additives, especially non-polar additives having relatively low molecular weight in relation to the polyamide polymer. The aesthetics of an article such as made by molding processes can be affected such as by migration, blooming or plate-out of the additives. On the other hand, manufacturers of polyamide-based molded and extruded goods strive to include low concentrations of additives for economic reasons.
Oxalic acid di-aryl anilides, formed on condensing the acid, partial ester or di-ester with one or two moles of aniline derivatives are known light stabilizers. Unsymmetrical oxalic acid dianilides are described, for example in U.S. Pat. No. 4,003,875.
U.S. Pat. No. 5,045,083 discloses polyamide, copper complex and oxanilide UV absorber.
U.S. Pat. No. 4,544,691 discloses stabilizer compounds containing UV-absorbing derivatives of oxanilide, benzalmalonates, .alpha.-cyanocinnamates and o-hydroxyphenylbenzotriazoles, having a reactive phenolic group(s). These additives are suggested for use with polymer coating materials.
U.S. Pat. No. 5,338,319 discloses a polyamide stabilization system containing a copper complex, and water soluble oxalic acid diaryl amide.
U.S. Pat. No. 5,380,774 discloses a polyamide molding composition containing a hindered phenolic antioxidant, phosphite, hindered amine stabilizer and optionally a benzotriazole.
European Publication number EP 0379470 discloses compounds used to improve the dyeability of polyamide. The compounds include a compound containing a sterically hindered amine group, trimesic acid trialkylamide, pyromellitic acid tetra-alkylamide, triamino substituted triazine, a hydroxyl-reactive dyestuff. Compounds containing a sterically hindered amine group are preferably ##STR1## where R is hydrogen, C.sub.1-8 alkyl, C.sub.1-8 alkoxy, or COR.sub.5.
WIPO publication WO 97/43335 discloses a stabilizer for polyamide having the structure: ##STR2## Polyamide polymers are characterized by high tensile strength, Young's modulus and abrasion resistance, however polyamides require stabilization against thermo-oxidative and photodegradation to sustain these properties in demanding use applications. Polyamides must accept lightfast dyeing systems in addition. Discoloration of the resin matrix is highly critical and may cause shade changes in colored articles, such as fibers and moldings of polyamide polymers. Therefore, further improvement in the stabilization systems for these materials is desirable in end uses such as fibers, and generally in high temperature processing of polyamide melts.
The polyamide structure contains labile hydrogens and exert reductive or basic properties which are problematic from the standpoint of colorants. For example, in polyamide, organic red dyes undergo significant loss of shade upon thermal processing at the typical melt processing temperatures for polyamide. It would be desirable to minimize the tendency of polyamide to exert this influence on dyes while at the same time protecting the polymer matrix from loss of the desired physical properties.
The evolution of polyamide stabilization system s has advanced from earlier recommended hindered phenolic types, for example Irganox.RTM. 1098, to a combination of an antioxidant and a benzotriazole, such as the combination of lrganox.RTM.1098 with Tinuvin.RTM. 234, well established I the art. Lightfast and photochemically stable dyed polyamide fibers containing oxalic acid di-aryl amides with a copper complex are known, for example from the aforementioned U.S. Pat. No. 5,338,319. These are applied in aqueous dye baths. In contrast, the present invention is directed to additives which are dispersed within the polyamide matrix by melt-processing at high temperatures above the glass transition, as in extrudates or moldings. WO 97/43335 provides a stabilizer which can be added in the melt to polyamide and improves heat stability, light stability, chemical stability, and dye affinity, however improvements are still sought.
Highly synergistic stabilization systems for polyamides have been found which exhibit unexpected further improvements for polyamide. These systems exhibit significant technical and economic advantages, even at low effective additive concentrations.